1. Field of the Invention
This invention relates to a protective film agent for use in laser dicing which applies a laser beam to a predetermined region of a wafer, such as a semiconductor wafer, to carry out predetermined processing, and a processing method by laser dicing using the protective film agent.
2. Description of the Related Art
As is well known among those skilled in the art, a wafer formed in a semiconductor device manufacturing process has a laminate, which comprises an insulating film and a functional film stacked on the face of a semiconductor substrate, such as silicon, partitioned by a lattice of scheduled division lines, called streets. Respective regions partitioned by the streets define semiconductor chips such as IC's or LSI's. That is, a plurality of semiconductor chips are obtained by cutting the wafer along the streets. An optical device wafer has a laminate, which comprises a gallium nitride-based compound semiconductor or the like stacked on the face of a sapphire substrate or the like, partitioned into a plurality of regions by streets. When cut along these streets, the optical device wafer is divided into optical devices, such as light emitting diodes or laser diodes. These optical devices find wide use in electrical equipment.
Such cutting of the wafer along the streets is usually performed by a cutting device called a dicer. This cutting device comprises a chuck table for holding the wafer, which is a workpiece, cutting means for cutting the wafer held on the chuck table, and moving means for moving the chuck table and the cutting means relative to each other. The cutting means includes a rotating spindle to be rotated at a high speed, and a cutting blade mounted on the spindle. The cutting blade comprises a disk-shaped base, and an annular cutting edge mounted on an outer peripheral portion of the side surface of the base. The cutting edge, for example, comprises diamond abrasive grains, which have a grain size of the order of 3 μm, fixed to the outer peripheral portion of the side surface of the base by electroforming, and is formed in a thickness of the order of 20 μm.
The wafer having the above-described laminated structure is a high brittleness material. Thus, the wafer has posed the problems that when the wafer is cut into semiconductor chips by the cutting blade (cutting edge), flaws, scratches or chipping occurs, causing the peeling of the insulating film required of circuit elements formed on the faces of the chips.
To avoid the above problems, it is currently becoming common practice to apply laser light along the streets prior to cutting with the cutting blade, thereby forming grooves commensurate with the width of the cutting blade (cutting edge), and then to perform cutting with the blade.
When laser light is applied along the streets of the wafer, however, the new problem has arisen that the laser light is absorbed, for example, into the silicon substrate, and its thermal energy leads to the melting or thermal decomposition of silicon, thus generating a silicon vapor, etc., which are condensed and deposited on the faces of the chips. The resulting condensation deposit (debris) of the silicon vapor, etc. markedly deteriorates the quality of the semiconductor chips.
To resolve the problem due to debris, Japanese Patent Application Laid-Open No. 1978-8634 (hereinafter referred to as Patent Document 1) and Japanese Patent Application Laid-Open No. 1993-211381 (hereinafter referred to as Patent Document 2) propose methods in which a protective film comprising a water-soluble resin is formed on a surface of a wafer to be processed, and this surface is irradiated with laser light via the protective film.
According to the methods of Patent Documents 1 and 2, the chip faces are protected with the protective film. Thus, even if a silicon vapor or the like, which is the thermal decomposition product of the substrate upon laser irradiation, scatters and condenses, its condensate (debris) deposits on the surface of the protective film, and does not deposit on the chip faces. Since the protective film is water-soluble, moreover, it can be easily removed by washing with water. That is, the debris on the protective film is washed away simultaneously with the washing of the protective film with water. As a result, deposition of the debris on the chip faces can be prevented.
With the foregoing methods, however, it is still impossible to prevent the deposition of debris completely, and the problem occurs that debris deposits on peripheral edge portions of the chips, in particular. The following mechanism may be involved: Upon irradiation with laser light, thermal decomposition of the substrate proceeds prior to the thermal decomposition of the protective film, and the pressure of a silicon vapor or the like, which is its thermal decomposition product, causes voids to be formed between the protective film and the peripheral edge portions of the chip faces (in the vicinity of the street lines) (in other words, partial peeling of the protective film takes place at the peripheral edge portions). As a result, debris deposition occurs at the peripheral edge portions of the chip faces. The problem also exists that the adhesion of the protective film to the wafer face is so low that the protective film is prone to peel off. Such peeling is another factor for easy deposition of debris on the peripheral edge portions of the chip faces.